I am wondering if a spinning permanent magnet's interaction with the Earth's magnetic field will produce a propelling force strong enough to propel a toy boat through the water. This propelling force would be most effective when the toy boat travels due North and stays on that course.

Please reference the drawing below. A permanent magnet would be fastened to a non-metallic bar that is perpendicular to an electric motor's shaft and on the other end of this bar would be a non-metallic counterweight. Say that the toy boat is 1 foot long, the magnet is a .5 inch cube N52 Neodymium magnet, the motor is spinning at 3000 rpm, and boat is in an indoor swimming pool.

The basic concept is that as the magnet is rotated from the position of facing due North to facing due South (180 degree rotation), the motor will have to push the magnet through the Earth's magnetic field and the reaction to this is that the toy boat will be pushed through the water. As the magnet is rotated from the position of facing due South to facing due North (180 degree rotation), the motor will not have to push the magnet through the Earth's magnetic field. So, during each full/360 degree rotation of the motor's shaft, the magnet's interaction with the Earth's magnetic field should create a paddle wheel effect that should propel the toy boat forward.

Could this magnetic paddle wheel effect propel this boat through the water?

Also, I am wondering if the rotational speed of the motor has a direct effect on the amount of propelling force that is generated. For example, will the motor have a harder time pushing the magnet through the Earth's magnetic field when it is spinning at 3000 rpm as compared to spinning at only 500 rpm?


I had to insert a revised drawing because the original drawing had the magnet positioned in the wrong orientation. Also added text to help convey concept.

enter image description here

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    $\begingroup$ Won’t the force be opposite when the magnet is at the top? $\endgroup$ – Solar Mike Jan 10 '19 at 17:21
  • $\begingroup$ @Solar Mike, well the way I see it is that the Earth's magnetic field will be pulling the magnet towards the magnetic South pole when it is at the top and the same thing will occur when the magnet is at the bottom. The overall effect should be that the boat is pulled/pushed towards the magnetic South pole of the Earth. $\endgroup$ – user18610 Jan 10 '19 at 17:47
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    $\begingroup$ You're dangerously close to expecting perpetual motion. The magnet interacts with the Earth's field in all orientations, not just at "top and bottom." There is no way you can extract more energy from the field than you have to put back in. $\endgroup$ – Carl Witthoft Jan 10 '19 at 18:46
  • $\begingroup$ Why wouldnt every electric motor then get a push from earth? $\endgroup$ – joojaa Jan 10 '19 at 20:38
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    $\begingroup$ The earths magnetic field is used to apply a torque to some satellites: en.wikipedia.org/wiki/Magnetorquer $\endgroup$ – Adam Jan 13 '19 at 21:08

Like anyone who has used a compass knows, earth's magnetic field does exert a force on magnetic objects. In theory this force could be harnessed to do work, but in practice it is too small to be a concern.

For example, a 1 meter long wire carrying 1 amp of current sitting perpendicular to earth's magnetic field will be subject to about 0.0001 Newtons of force. That's not going to be enough to overcome friction let alone build up any noticeable amount of speed. For comparison, dropping a paper clip off the side of a table exerts 100 times that force.

As to your "paddle wheel": The magnet is always in earth's field and is the equivalent of a paddle wheel that is fully submerged in the water. That means the force exerted on your spinning magnet will follow a typical sinusoid pattern, resulting in net zero work. You would be better off just leaving the magnet stationary, but don't expect your boat to move a noticeable distance unless you see all the iron objects in the world flying towards the poles.

  • $\begingroup$ well not quite, the magnet has no charge since the permanent magnet has both positive and negative charge so while it can easily orient itself it can not be used to generate power. The magnet needs to be in,a area where the field is starting to change abruptly so that the ends dont experience same force. $\endgroup$ – joojaa Jan 11 '19 at 14:18
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    $\begingroup$ Not power, force, sorry. $\endgroup$ – joojaa Jan 11 '19 at 14:27
  • $\begingroup$ @joojaa to nitpick, any field strength change will work, albeit moving the magnet slower than the famous pitchblende drop experiment :-) $\endgroup$ – Carl Witthoft Jan 11 '19 at 17:28
  • $\begingroup$ @CarlWitthoft but offcourse, just that the magnetic field on earth is locally constant. It is better to make the shaft even slightly slanted and use it as propeller. ;) $\endgroup$ – joojaa Jan 11 '19 at 17:49

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